1. Field of the Invention
This invention relates to the production of biaxially stretch oriented thermoplastic flexible film utilizing the "bubble" method and particularly to an automated mechanical device the for production of the isolated bubble.
2. Description of the Related Art
The process of biaxially orienting films from a variety of thermoplastic materials is well known as are the advantages of the biaxially oriented films so produced. Biaxially oriented films of polypropylene, high-density polyethylene, polystyrene, and polyvinylidene chloride have achieved substantial commercial recognition.
The "bubble" method of biaxially orienting tubular films is also well known in the art and has achieved substantial commercial recognition. For example, U.S. Pat. No. 3,456,044 (Pahlke) teaches one type of bubble method by disclosing an apparatus for the production of biaxially oriented film and this type of method is called the "double bubble" method. U.S. Pat. No. 3,555,604 (Pahlke) teaches the biaxially oriented film produced using the apparatus of U.S. Pat. No. 3,456,044 (Pahlke) using the "double bubble method". Another type of bubble method for the production of a film, such as a saran film, using the "trapped bubble" method is known for example from U.S. Pat. No. 4,112,181 (Baird) and U.S. Pat. No. 3,741,253 (Brax et al.). By saran, is meant copolymers of vinylidene chloride, and it is noted that the term "saran" has become generic in the United States for such copolymers. The disclosures of all of these patents are incorporated herein by reference.
By the "bubble" method of biaxially orienting thermoplastic film is meant a process in which a primary tubing is first formed by melt extrusion from a die, inflated by the admission of air, cooled, and collapsed. In the double bubble process, this first stage also involves a bubble, typically referred to as the primary bubble, which is cooled and collapsed as a tape. But in the trapped bubble process, this first stage does not involve a bubble, so the primary is typically referred to as a primary tape. Regardless, in the second stage, the collapsed tube (typically called a tape) is then reinflated to form an isolated bubble. The tubing is advanced through a heating zone to raise the film to its draw temperature. In the double bubble process, the heating is directly onto the inflated isolated bubble in the expansion zone, whereas in the trapped bubble process the heating is directly on the collapsed tape, such as via a hot water or hot air tank, prior to inflation of the isolated bubble in the expansion zone. In a draw or expansion zone the tubing is radially expanded in both the transverse and machine directions at a temperature such that orientation occurs in both directions. The material is then rapidly cooled to set the orientation.
Typically, the drawing of this isolated bubble is done by a person by threading the formed, inflated, cooled and deflated thermoplastic tape through a first set of rolls, which are then closed up (or down in some bubble processes) through a second set of rolls, in an open position, located some distance away from the first set of rolls and then grabbed by a human operator. The human operator inserts a high flow air hose into the end of the tape, seals the tape around the nozzle end of the air hose and then using low flow pulses of air inflates the tape from the point where the tape is attached to the air hose to the closed first set of rolls. Next the human operator starts the bubble by pulsing air, typically high flow air but low flow air is sometimes used, or if the thermoplastic tape is not elastic enough, yanks with force on the tape while simultaneously pulsing high and/or low flow air. Once the bubble is started the human operator must continue pulsing high pressure air into the tape to build the bubble to the required size. Because the tape is expanded from about 3 to about 7 times in both the transverse and machine directions, the length of the tape also increases from 3 to 7 times requiring the human operator to move back away from the film orienting machinery. Further, because the human operator must constantly pulse high flow air and visually monitor the bubble formation, the human operator is forced to walk backwards away from the film orienting machinery staying clear of the high flow hose trailing behind him or her. The speed of the whole operation is therefore determined by the speed at which the human operator can create the bubble while moving backwards. This requires a strong, agile, skilled human operator to create a bubble of desired size and at the fastest possible speed. This operation is also one of great concern from a safety standpoint in that it requires a great distance of clear space for the human operator to move, and requires the human operator to move backwards without being able to see where he or she is going. Also, large companies, which can afford it, have the second set of rollers on a vertically movable elevator in order to adjust bubble size after a bubble is brought up by the human operator. But small companies cannot afford the elevator and so the second set of rollers is vertically stationary, so if the brought up bubble is the wrong size, the human operator must bring it down (break it) and start over.